Phosphate esters are a class of chemicals that includes numerous chemical warfare agents such as VX, pesticides, and insecticides, further including paraoxon and parathion. Each of these compounds includes a phosphate ester bond, and irreversibly blocks a serine hydroxyl group within the enzyme acetylcholinesterase by phosphorylation, resulting in a disruption of a cell's neurologic function. Thus, many organophosphate compounds can be neurotoxic. Further, phosphate esters used as pesticides can be toxic to animals and also pollute soil and water. The creation of phosphate esters for chemical warfare and use as pesticides results in the need for a safe and effective process of degradation in order to completely eliminate the compounds without persistent toxic environmental and medical effects. Thus, it has been desirable to find methods of degrading organophosphate compounds. Much of the work in finding suitable methods for degrading phosphate esters in pesticides originates from the research done to degrade phosphate ester nerve agents.
Sulfur-containing containing organophosphates are used as systemic neurotoxins for agricultural and residential pesticides. For example, diazonin (thiophosphate) was used widely in the United States as an insecticide against cockroaches, silverfish, ants and fleas. It is also used to control pest insets in soil, on plants and on fruit and vegetable field crops. Prior to a residential ban (2004) on residential use, diazonin was heavily used and it was estimated up to 80% of all residential lawn and garden insecticides contained diazonin (13 million pounds/year). 10 Due to its heavy use, diazonin is prevalent in the environment. The US Geological Survey National Stream Water Quality Network found diazonin in all major U.S. river systems including the Rio Grande, Mississippi, Columbia and Colorado at levels exceeding the safety limit for aquatic organisms.
Like all organophosphates, diazonin is an acetylcholine esterase inhibitor that acts as a neurotoxin. In humans diazonin overstimulates the nervous system that may result in nausea, dizziness and confusion, and high levels it can cause respiratory weakness, headaches and death. Diazonin is listed as a developmental toxin in the EPA's Toxics Release Inventory. Another similar thiophosphate pesticide is parathion which is still in agricultural use. The US EPA considers parathion as a possible human carcinogen 12 and the non-governmental organization Pesticide Action Network (PAN) considers parathion as one of the most dangerous pesticides. It was used as a chemical warfare weapon by the Selous Scouts during Rhodesian Bush War (1964-1979). The WHO and PAN propose a general and global ban on parathion; its use is banned and restructured in 23 countries and its import is illegal in 50 countries. Methyl parathion is approved for use as an agricultural pesticide but in 1999 the EPA canceled its use on food crops consumed by children such as apples, peaches, pear, carrots and peas. It is allowed in other crops such as vegetables, nuts and grains that may be eaten by humans.
In addition to targeting the pesticides, there is a need to degrade military organophosphate neurotoxins. O-ethyl-S-[2-diisopropylamino)ethyl]methylphosphonothioate (commonly referred to as VX) is such a chemical warfare agent. Symptoms of exposure to VX include coughing, difficulty breathing, sweating, vomiting, urination/defecation, headache, tremors, unsteadiness and confusion, ultimately progressing to death. It is the most lethal human-made neurotoxin. The United States has a stockpile of thousands of tons of VX that must be destroyed to comply with the Chemical Weapons Treaty of 1997. In addition, Russia is also known to possess quantities of VX.
Traditionally, VX is degraded on a large scale by hydrolysis with concentrated aqueous sodium hydroxide resulting in competing cleavage of the P—S and P—O esters, with approximately 87% P—S bond cleavage and 13% P—O bond cleavage. This is problematic because the byproduct of the P—O bond cleavage, S-[2-(diisopropylamino)ethyl]methylphosphonic acid, has a toxicity comparable to VX and requires additional steps such as oxidative pretreatment for destruction. Caustic neutralization at 90° C. (16.6 wt. % VX, 8.8 wt. % NaOH, 74.6 wt. % H2O) produces a similar ratio of bond cleavage, but allows S-[2-(diisopropylamino)ethyl]methyl-phosphonic acid to be broken down concurrently producing methyl phosphonic acid and thiolamine. However, this process requires specific control over both the pH and temperature of the reaction to ensure no byproducts are produced.
There is a need for a method to selectively cleave the P—S bond of the phosphate ester VX to eliminate the toxic byproducts of its degradation, so as not to require further degradation. Various additional aqueous compounds have been used in the degradation of VX, but are either unsuccessful at selectively cleaving the P—S bond or present commercial difficulties in their ability to be used in mass quantities. For example, aqueous potassium peroxymonosulfate selectively cleaves the P—S bond in VX. However, the solubility of potassium peroxymonosulfate is limited at low pH and the oxidant decomposes at any pH above 5. Alternatively, the use of potassium peroxymonosulfate in polar organic solvents generates a toxic diphosphonate as a major byproduct.
Other degradation methods for phosphonothioates include incineration and oxidation with peroxides. Incineration is a politically unacceptable degradation method which is no longer actively pursued by the United States. Alternatively, hydrolytic degradation of phosphonothioates lacks selectivity and results in both P—O and P—S degradation pathways, resulting in toxic byproducts.
The known methods to degrade phosphate ester pesticides include hydrolysis by microorganisms, degradation or hydrolysis by Cu (II), Hg(II) and clays, surface catalyzed hydrolysis by Al2O3, TiO2 and FeOOH (goethite), and hydrolysis by Rh(III) and Ir(III) coordination complexes that are overly expensive.
Still another method of degrading phosphate esters includes a heterogeneous-phase method that targets organophosphates in a passive approach binding or sequestering the neurotoxins. One of the more convenient approaches to this is with smetic clay that can actively degrade these phosphate ester pesticides. However, degradation by smetic clay takes up to one year.
Therefore, an immediate problem that must be addressed is a convenient and benign way to degrade these ubiquitous organophosphate pesticides and chemical warfare agents. Moreover, the method and material has to be easily made, stable in most environmental conditions, facilely recycled and target against a general array of sulfur-containing organophosphate pesticides.
There is also a need to recycle the organophosphate product for phosphorus recovery, which has been identified as a national priority by the National Science Foundation. The United States and China have an estimated 30-year domestic supply of minable phosphorus. Therefore, by the middle of this century the world's two largest economies may run out of affordable supplies of domestic phosphorus. Europe has labeled phosphorus as a “critical material,” and it will soon compete with the U.S. and China for the dwindling phosphorus supply. The need for phosphorus recovery is best summarized by a quote from Isaac Asimov where he said “life can multiply until all phosphorus is gone, and then there is an inexorable halt which nothing can prevent. . . . We may be able to substitute nuclear power for coal, and plastics for wood, and yeast for meat . . . but for phosphorus there is neither substitute nor replacement.”
Prior heterogeneous-phase methods that target organophosphates take a passive approach that bind or sequester the neurotoxins; perhaps the most convenient material is smetic clay that actively degrades these pesticides. However, degradation by smetic clay takes up to one year. The supported molybdenum polymers of this invention effectively degrade these pesticides in a period of days. Moreover, in this invention the active species is regenerated with the addition of H2O2 and is catalytic in the molybdenum metal immobilized on the support. A prior example using a polystyrene ammonium fluoride successfully degrades VX but in a non-catalytic fashion and goes through a toxic phospho fluoridate (i.e., Sarin) intermediate.
Additionally, the prior methods of degrading organophosphate compounds has not provided the ability to recover useful chemicals (such as phosphorus) from the degradation process. Rather the approach has focused on degrading the compounds such that they are capable of disposal.
Thus, there is a need for new methods of degrading organophosphate compounds. Further, there is a need for degradation methods that can result in the recovery of useful chemicals, such as phosphorus.
Accordingly, it is an objective of the claimed invention to develop
A further object of the invention is to provide a method of degrading phosphate esters in a shorter time period and under more mild conditions.
Still a further object of the invention is to provide a method of degrading phosphate esters that can result in the recovery of useful phosphorus-containing compounds.
Other objects, advantages and features of the present invention will become apparent from the following specification taken in conjunction with the accompanying figures.
Various embodiments of the present invention are described in detail with reference to the figures. Reference to various embodiments does not limit the scope of the invention. Figures represented herein are not limitations to the various embodiments according to the invention and are presented for exemplary illustration of the invention.